1. Field of the Invention
The present invention relates to porous refractory plugs adapted to be positioned in walls of aluminum melting or holding furnaces for introduction into molten aluminum of gases for fluxing and other purposes. More particularly, the present invention relates to an improved porous refractory plug structure that has significantly improved service life by providing additional protection for metallic components of such plugs through the use of ceramic coatings.
2. Description of the Related Art
The introduction of gases into molten aluminum was previously performed by inserting into the molten aluminum a metal pipe, sometimes referred to as a wand, and causing the gases to flow through the pipe and into the molten metal. The gases were provided to remove from the melt undesired components, such as inclusions, dissolved hydrogen, certain metallic elements such as sodium and lithium, and the like. Because of the sizes of the common melting and holding furnaces, which can be as much as thirty or more feet across, the wand technique is relatively inefficient because the gas that issues from the wand contacts only a small portion of the molten metal, and the wand must therefore be moved about the molten metal mass so that substantially the entire mass of molten metal is exposed to and contacted by the gas.
One way in which the dispersion into a molten metal of fluxing gases can be more effectively accomplished is by providing a series of refractory porous plugs in the furnace wall, most commonly in the bottom wall of the furnace. Each plug is in communication with a source of fluxing gas, which passes through the plug and directly or indirectly into the molten metal mass. By providing a sufficient number of such porous plugs distributed over the furnace bottom, fluxing of the melt can be more quickly and more efficiently performed. Early refractory porous plug structures are disclosed in U.S. Pat. No. 3,834,685, which issued on Sep. 10, 1974, to Leonard P. Ziemkiewicz, et al., and in U.S. Pat. No. 4,053,147, which issued on Oct. 11, 1977, to Robert Moser et al.
One form of porous plug structure that has been found to be particularly suitable for use in aluminum melting and holding furnaces is disclosed in an article entitled, "A Two Year User's Experience with Porous Plug Fluxing in a Modern Casting Facility," published in Light Metals 1993, edited by Subodh K. Das, published by The Minerals, Metals, and Materials Society, copyright 1992. As disclosed in that article, the porous refractory plug structure includes a frustoconical central porous core member that is surrounded at its sides and bottom with a non-porous refractory lining material. Positioned around an outer portion of the non-porous refractory material is a metallic protective lining, sometimes referred to as a "can," for confining a fluxing gas that enters the central porous core member and for preventing contact of the fluxing gas with the adjacent refractory furnace material. Inconel is disclosed as the material from which the metallic lining is made, and because inconel, a nickel-based alloy, has well-known corrosion-resistant properties it was thought that long life of the porous plug structure was assured.
Recent experience with such porous refractory plugs has shown that the inconel liner, despite its intrinsic corrosion resistance, is in time subject to deterioration by certain fluxing gases such as chlorine, which can reach the inconel liner through cracks that can occur in the non-porous refractory liner. Further, although the effective service life of such plug structures is of the order of about 2 years, it was desired that the effective service life be further increased, to reduce the servicing interval at which replacement of the plugs was required. It has been shown that the ceramic coating bonds to both the liner material and the adjacent refractory, effectively sealing this joint and impending the flow of flux gas along this surface.
It is an object of the present invention to overcome the shortcomings of the prior art porous refractory plug structures.
It is a further object of the present invention to provide a porous refractory plug for use in aluminum melting and holding furnaces wherein the plug structure has an operating life of at least about three years.